The Law of Definite Proportions states that elements in a compound are always present in the same proportions by mass. The percent composition is the relative mass of each element in a compound. The mass percent is the mass of an element in a compound expressed as a percentage of the total mass of the compound. For example, a water molecule has a composition of 2 hydrogen atoms and 1 oxygen atom. The mass percent of hydrogen is 11.2% and the mass percent of oxygen is 88.8%. These numbers together make up the percent composition of a water molecule. Thus, for any pure sample of water, it will always have this percent composition. That is, water will be 11.2% hydrogen and 88.8% oxygen.
Calculations
Finding the percent composition from mass data
Example: If a compound has a total mass of 34.37 g, where 12.94 g is magnesium and 21.43 g is phosphorous, what is the percent composition?
Mass percent of Mg = 12.94g/34.37g = 37.6%
Mass percent of P = 21.43g/34.37g = 62.4%
Therefore, the percent composition of this compound is 37.6% magnesium and 62.4% phosphorous.
Calculating Percent Composition from a Chemical Formula
Example: What is sodium chloride, NaCl’s percentage composition?
Assume a sample size of 1 mole so that the mass of NaCl is 58.44 g (since molar mass is 58.44 g/mole). This would mean the masses of Na and Cl are 22.99g and 35.45g respectively.
Mass percent of Na = 22.99g/58.44g = 39.3%
Mass percent of Cl = 35.45g/58.44g = 60.7%
Therefore, the percent composition of NaCl is 39.3% sodium and 60.7% chloride.
This article was written for you by Samantha, one of the tutors with SchoolTutoring Academy.